bounded loop support

[brendangregg]

Great work developing this!

I see it uses many unroll() loops. Linux 5.3 added bounded loops, which would greatly reduce the instruction count bpftrace generates, plus allow us to implement for and while loops. See bpftrace/bpftrace#872

Once that's done, bpftrace-tetris can either use the new loop syntax (for, while), or at least, the generated instructions from unroll() should be a lot less. Just adding this ticket as a reminder that we do need to do issue 872 on bpftrace.